Conventionally, nuclear reactors comprise pressure tubes or pressure vessels, though nuclear reactors that have both pressure tubes and a pressure vessel have been disclosed. The present invention is especially applicable to those having pressure tubes.
At present, nuclear power plant safety is of particular concern in the aftermath of the Fukishama accident in Japan in 2011 and others in which the fuel becomes exposed after the reactor has shut down. Some proposals for improving safety have focused upon prolonging the time to catastrophic failure in a severe accident in which decay heat may cause either or both of fuel cladding failure and hydrogen production. One option is to provide passive decay heat removal following a severe accident but this requires the fuel cladding to be capable of retaining the fuel fission products while the decay heat is transferred from the fuel, predominantly by thermal radiation.
Although improving safety is of paramount importance, it is desirable to do so without reducing efficiency. In fact, there is an ongoing desire to improve the efficiency of nuclear reactor power plants without prejudice to their safety. Improved efficiency can be obtained by increasing operating temperatures. To this end, it has been proposed to build nuclear power reactors employing supercritical water which requires much higher operating pressures than the current art PWR, BWR and PHWR. A fuel must be capable of operating at the temperature and pressure of supercritical water, and withstanding the corrosive environment of irradiated supercritical water and radiation damage. The fuel sheath or cladding must also have acceptably low neutron absorption to function economically while desirably providing for passive decay heat removal following a severe accident in which the fuel becomes exposed after the reactor has shut down.
In this industry, a variety of terms are used for the pressure barrier between the fuel and the reactor coolant. For convenience, in the context of this specification, the term “cladding” will be used for such pressure barrier, whether in a pressure-tube or pressure-vessel type of reactor.
Also, the term “fuel elements” will be used to embrace both the fuel elements of a pressure-tube type of reactor and the fuel rods of a pressure-vessel type of reactor.
The term “fuel assembly” refers to a plurality of fuel elements which are held together in parallel. In the case of a PHWR, this fuel assembly usually is called a “fuel bundle”.
Moreover, the term “inert” material will be used to mean that the material does not generate significant hydrogen in the presence of steam or corrode significantly in the presence of irradiated reactor coolant water.
It is known to use supercritical water systems in fossil fuel power stations. However, the technologies, particularly materials, used in supercritical fossil fuel stations cannot necessarily be used in supercritical nuclear reactor stations where low neutron absorption and corrosion resistance at supercritical temperatures and radiation levels are particularly important. This is especially so for the fuels and the fuel assemblies containing them.
It is known, for example, to use stainless steel to clad fuel for a higher temperature operation. It is unlikely that, in severe accident conditions, this fuel cladding would have been capable of retaining fission products while passively transferring decay heat in a PHWR, PWR or BWR.
Other steels, nickel and titanium-based alloys that have been studied for supercritical water reactor use also have relatively high neutron absorption and entail the use of enriched uranium. They would not be entirely suitable for use in applying similar reactor physics when refitting an existing reactor, for example a PHWR.
Current pressure tube type heavy water reactors use a natural enrichment uranium dioxide fuel in a fuel bundle located inside a zirconium alloy pressure tube. The fuel bundle typically comprises 28, 37 or 43 fuel elements. Heavy water coolant inside the pressure tube surrounds the fuel bundles.
The fuel elements are held in a bundle configuration by welding to end plates. Zircalloy bearing pads are located on the outer ring of elements. Spacers are located on the faces of the elements that are adjacent to neighbouring elements i.e., that are juxtaposed when the fuel elements are assembled into a bundle.
Typically, each fuel element comprises a plurality of cylindrical uranium dioxide fuel pellets inside a tubular zircalloy fuel cladding capped at the ends with zircalloy end caps welded to the cladding. Some fuel elements may have a layer of a graphite-based mixture between the fuel elements and fuel cladding.
The fuel operates by producing heat and neutrons from the fission of uranium in the fuel pellets. The rate of heat release is controlled by reactivity mechanisms that control the population of neutrons in the reactor core at any given time. The heat produced in the fuel element is conducted outward to the outer surface of the fuel element where it crosses a small gap or contact area to the inside surface of the fuel cladding. The heat is conducted through the wall of the fuel cladding and convected away into the coolant, which conveys it away as “useful” heat. At least some of the remaining “waste” heat not conveyed away by the coolant passes through the pressure tubs and/or surrounding calandria tube, as the case may be.
When the reactor is shut down, a lower rate of fuel heating continues to be present from the decay of fission products. This heat must be removed to prevent the fuel from heating up to a high temperature at which the fuel cladding will fail and, ultimately, the fuel may melt. To remove this heat, the coolant is kept flowing at all times, which typically requires the continued availability of electric power and a pump that is always operational. If the coolant flow ceases or reduces unacceptably, (known as loss-of-flow accident (LOFA)) or the coolant inventory is lost (known as loss-of-coolant accident (LOCA)), the fuel will heat up and eventually release hazardous fission products. More particularly, in the event of a cladding failure, hazardous gaseous radioactive fission products are promptly released to the coolant. When zirconium, in particular, gets hot in the presence of steam, it forms hydrogen gas which can cause explosions.
While known fuels may function satisfactorily at the current sub-critical pressures and temperatures, their cladding lacks the strength and corrosion resistance to operate satisfactorily at higher temperatures and pressures and especially at the much higher supercritical pressure and temperatures that allow increased power output and thermal efficiency. In severe accident conditions, the cladding could heat up, oxidize (producing hydrogen) and melt before one can get the decay heat out of the fuel channel.
This problem is exacerbated by insulating the pressure tube to ameliorate heat loss. For example, it is known to insulate the pressure tube by providing a surrounding tube around the pressure tube, known by those skilled this art as a “calandria tube”, and filling an annular cavity between these tubes with an insulating gas.
It has been proposed, in a paper by Yetizir, M., W. Diamond et al., “Conceptual Mechanical Design for a Pressure-Tube Type Supercritical Water Cooled Reactor”, The 5th International Symposium on SCWR, Vancouver, Canada, Mar. 13-16, 2011), and a document referenced therein, to use a solid insulator within the pressure tube. Yetizir et al. use a zirconium alloy, specifically identified by Yetizir et al. as “Excel”, with low neutron cross-section for the pressure tube and line the pressure tube with a porous ceramic insulator itself lined with stainless steel. With this arrangement, the exterior surface of the pressure tube is in contact with the coolant moderator and its interior surface is insulated from the fuel and coolant by the stainless steel lined ceramic insulator. The stainless steel lined ceramic insulator is yttrium-stabilized zirconia (YSZ) which has low neutron absorption properties and good, if not excellent, thermal resistance. As a result, the pressure tube tends to remain at the temperature of the coolant and is less likely to rupture. Nevertheless, neither of these insulated-pressure tube arrangements is entirely satisfactory, since decay heat will cause the fuel to heat-up to very high temperatures because of the thermal insulating effect of the stainless steel lined insulator or the insulating gas in the annular cavity. Under severe accident conditions, the temperature will become high enough to cause the conventional fuel cladding to oxidize and eventually melt, leading to hydrogen production and release of fission products. Similar considerations apply to fuel cladding in pressure vessel reactors under severe accident conditions.